Rewrite URL for only archive page (custom post type)

You can have a separate slug for the post type archive by setting the slug as the value for the has_archive argument, instead of just true:

register_post_type(
    'event',
    array(
        'has_archive' => 'agenda',
        // etc.
    )
);

Now you can use single-event.php and archive-event.php for the single and archive views, but the URL for the archive will be siteurl.com/agenda.